Power MOSFET's are often implemented as an array of switching cells (perhaps 10,000 in number) formed on a single chip and connected in parallel. Such devices are used in electronic circuits for switching and controlling electrical power to desired loads. In such applications, it is often desirable to sense the current through the device and the load, the voltage across the device, the power dissipated in the device, and the temperature of the device. The results of such sensing can be used to detect device and load efficiency, short circuit conditions, meltdown conditions, etc.
Voltage sensing has typically consisted of connecting external sensing circuitry to the high voltage lead of the power chip (for example, the drain in the case of a MOSFET). However, such sensing circuitry may affect the performance of the switch or load, may have poor reliability or accuracy, and tends to use expensive components. In addition, the circuitry should be gated and protected from the high voltage transients of the switch and load, which requires even more complex and expensive circuitry.
Current sensing has typically been accomplished by using a power resistor in series with the power switching device or the load, and sensing the voltage across the resistor. However, power resistors dissipate large amounts of power, affecting the performance of the switch and load, and resulting in excess heat and inaccurate results. Although some of these power dissipation problems can be overcome by utilizing 5-watt resistors in the 10-20 milliohm range, such resistors are expensive and difficult to make. Magnetic coils have also been utilized to sense the current via magnetic induction. However, such coils are inaccurate and insert parasitic inductance into the circuit, again affecting the accuracy of the data.
Power dissipation and resistance of the power chip have been sensed by using a combination of prior art voltage and current sensing techniques. This has all the disadvantages of both the voltage sensing techniques and the current sensing techniques discussed above.
Temperature sensing has typically been accomplished utilizing a forward biased p/n junction on the power chip. Such a junction requires a separate current source which complicates the design and construction of the power chip. Moreover, the sensing is usually done when the main device is off. In order to sense temperature when the device is on, more complex circuitry is needed either on or off the chip.
More recently, the "current mirror" technique has been introduced for current sensing in power MOSFET's. In this technique, a small number of the cells on the chip ("probe cells") have their terminals connected in common with each other, but separate from the terminals of the remaining cells. The current flowing through these probe cells represents a small fraction of the total current flowing through the main portion of the chip. This current can be measured by measuring the voltage across a resistor placed in series with the probe cells. Since the current is so small, a larger valued standard resistor can be used. The current mirror technique does not affect the performance of the switch or load because very little power is drained from a few cells. However, problems due to crosstalk between the probe cells and switching cells have limited the accuracy of the technique.